@Dallas 2000 years after revolution
The clans come together and ascend from the caves with oil driven mechs to soon find the dreaded foe from the age of legend that was inscribed on the walls, as a new civilisation in a fight for control over their old home, now a cyber-apocalyptic wasteland. The inner shell of the sphere used as a storage and training facility for the intergalactic combine empire with industrial mechs of their own - centuries after the red letter.
Sheeiiiiit. This paragraph alone gives me an idea for a whole new HL2 related story. It would take place a couple centuries or even thousands of years after the uprising when humanity finally beat back the Combine, but eventually had to go into hiding because they couldn't fight the hordes of Xenian wildlife taking over the planet. After time the human civilizations start to regress into a semi-mystical tribal society that still retains some advanced technology reverse-engineered from the Combine but only a select group of people know how to use and maintain it, and these guys are revered like royalty and treated as though they have some supernatural powers. During that same period Freeman basically disappeared and no one knows what happened to him, and it's been so long that no one knows for sure whether he was a real person or not, but he's worshipped as a God by the humans. The vortigaunts would be like the scribes and record keepers of these new societies since they all share a semi-gestalt consciousness and can more or less remember what happened throughout the ages, though they're seen as kind of aloof weirdos that no one really wants anything to do with, yet they're still respected because of their near-supernatural aspects.
Eventually a few of the more advanced tribes of mankind form together and move back to the surface to take their world back from the Xen creatures that claimed it, but at the same time the Combine makes its emergence as well. When the uprising ended the Combine consolidated what resources they had left and also went into hiding, forming a semi-feudal society with the advisors at the top rung, and the more extremely loyal human collaborators below them basically functioning as lords of their own little sectors with their own armies. The technology used to create transhuman soldiers would have been lost to them over time, so what little transhuman soldiers are still left have been upgraded to become one-man killing machines that can take on entire squads by themselves. The rest of Overwatch's forces would consist of slave soldiers or a more well-trained and hardened core of loyalist troops. I can imagine that over the centuries the loyalists would've started like "houses" and began to see service to the Combine as an honor and a privilege, and so the tradition of martial service for the Union would be passed down to each generation. The human loyalists would see their administrators as kings and queens while revering the advisors as gods, and any word passed down from the administrators would be considered the word of the gods since it came straight from the advisors.
An apocalyptic struggle between two cyber/dieselpunk factions of fanatical zealots and hostile invasive life in a world that's been terraformed to the point of being unrecognizable.
I will make this happen, so help me god. Whether anyone wants to help or not.